The present invention relates to a novel process for preparing highly pure ammonium paratungstate hydrates and ammonium paratungstate decahydrate having a selected crystal structure.
Ammonium paratungstate hydrates (hereinafter referred to as APTs) are known intermediates for producing tungsten metal, tungsten-containing catalysts or hard materials based on tungsten, for example tungsten carbides.
The preparation of highly pure APT is carried out essentially via acid or alkali digestion or fusion of tungsten-containing concentrates or tungsten scrap with subsequent purification stages involving precipitation processes and liquid-liquid extraction. The purified solution is generally concentrated by evaporation, resulting in APT finally crystallizing out. The publication “Kristallographische Untersuchung der Ammoniumparawolframate (Zeitschrift für Kristallographie, vol. 120, pp. 216-228 (1964))” by M. Hähnert states that APT×10H2O is formed on slow evaporation of an ammonium tungstate solution prepared from WO3 and NH3 solution. The crystals have an acicular morphology. The bulk density of the product is 0.7 g/cm3. This is not an industrial process but a preparative laboratory process.
Further routes to the preparation of APT×10H2O are described in the article “Characterisation of various commercial forms of ammonium paratungstate powder, Journal of Material Science, No. 10 (1975), pages 571-577”.
In one process variant, an ammonium tungstate solution prepared from tungstic acid and an excess of NH3 solution is subject to freeze drying. The product formed has a crumbly poorly defined crystal morphology and the bulk density is 1.03 g/cm3. In a second process variant, an ammonium tungstate solution prepared as described above is slowly neutralized with HCl solution at room temperature. The acicular crystals formed have a length of 16-70 μm and a width of 2-14 μm and the bulk density is 1.46 g/cm3. Both processes form APT×10H2O having a low bulk density and the necessary product purity can be achieved only when using clean tungstic acid. In addition, a freeze drying procedure in one case and the use of clean HCl solution in the other case leads to very high process costs. When HCl is used, the product is contaminated with chloride as a result of the process.
In the specialist book “Metallurgie der seltenen Metalle, Seligman, Krejn and Samsonov (1978), Metallurgia publishers (USSR), pp. 62-63”, an industrial process for preparing APT×10H2O is described as follows. Scheelite concentrate is digested with HCl solution at 90-100° C. so as to form tungstic acid. The tungstic acid is subsequently dissolved in NH3 solution and the solution is cooled. The resulting ammonium tungstate solution is slowly neutralized with HCl solution to a pH of 7.3-7.4 while stirring. After allowing to stand for 24 hours, the APT×10H2O product is separated off. The acicular crystals have a length of 15-25 μm and a width of 1-3 μm and the bulk density is 0.98 g/cm3. The crystallization yield here is 85-90%. This product still contains considerable amounts of impurities. A further disadvantage of this process is the high consumption of clean HCl solution and the large amount of W-containing NH4Cl solution (mother liquor) which has to be worked up.
FIG. 1 depicts a scanning electron micrograph (SEM) of a product prepared by this process.
It is known that clean ammonium tungstate solutions are used for preparing highly pure W salts. These solutions are usually prepared industrially by sodium hydroxide or sodium carbonate fusion of W concentrates and subsequent precipitation of P, As, Si and Mo impurities by addition of Mg, Al salts and sodium hydrogensulfide and then carrying out a liquid-liquid extraction using amine-containing organic phases. The parasitic formation of APT×10H2O when a liquid-liquid reextraction with NH3 solution is carried out is mentioned in U.S. Pat. No. 4,450,144 and U.S. Pat. No. 4,092,400. However, the aim of these processes is to prepare clean ammonium tungstate solution which can be converted by evaporative crystallization into APT×4H2O. The formation of APT×10H2O in the reextraction in the processes mentioned has an adverse effect on the phase separation, the purity of the APT×4H2O product and the crystallization yield. For this reason, the abovementioned patent texts describe possible ways of reducing or preventing the formation of APT×10H2O crystallites in the reextraction.
A process for preparing APT via digestion of W-containing concentrates with subsequent liquid-liquid extraction of the tungsten compounds and subsequent reextraction with NH3 solution is described in DE-B-1,150,962. Here too, an organic amine phase (tertiary alkylamine) is used for separating tungsten from W-containing digestion solution. According to this process, as can be seen from the accompanying example, the organic amine phase laden with 23-27 g/l of tungsten is placed in a settling apparatus in the form of a long tower and reextracted by dropwise addition of 5-29% strength NH3 solution. The reextraction is carried out at an NH3:W molar ratio of the starting solutions in the range from 3.6:1 to 50.1:1, depending on the embodiment, and a ratio of organic phase (OP) to aqueous NH3 solution in the range from 2.1:1 to 5.5:1. The APT product formed is subsequently filtered off and dried. Carrying out the reextraction by this process leads to a finely crystalline acicular APT×10H2O product having OP adhering to the surface and a low bulk density of <1.0 g/cm3. FIG. 2 depicts a scanning electron micrograph of a product prepared by this process. Chemical analysis of the product shows a high proportion of carbon contamination of 5000-10 000 ppm. For these reasons, the material is not suitable for further processing steps. In addition, the phase separation in the reextraction as described in DE-B-1,150,962 occurs only after long standing. This can be attributed to the finely crystalline character of the product. Owing to the high W content of the mother liquor, which is due to the NH3:W molar ratio used in the reextraction, the crystallization yield in this process is not more than 65% (see abovementioned publication, experiment 4). The poor product quality, the poor phase separation and the low crystallization yield has resulted in this process not having been implemented to the present time.
For these reasons, the further developments of W reextraction from amine-containing organic phases have gone in the direction of liquid-liquid reextraction with avoidance of APT precipitation and subsequent APT production by evaporation of the clean ammonium tungstate extract solutions, as described in the abovementioned documents U.S. Pat. No. 4,450,144 and U.S. Pat. No. 4,092,400.